Method and system for allocating communication resources

ABSTRACT

The present invention relates to a method for allocating communication resources in a multi-user cellular communication system, wherein communication resources are divided in time periods and frequency sub-bands, wherein part of the communication resources are used for frequency-localized communication channels, and part of the communication resources are used for frequency distributed channels The method further comprises the steps of: classifying part of the frequency sub-bands as frequency sub-bands carrying frequency-distributed channels, classifying the remaining part of the frequency sub-bands as frequency sub-bands carrying frequency-localized channels. The present invention also relates to a system, a transmitter and a communication system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/920,760, filed Jun. 18, 2013, which is a continuation of U.S. patentapplication Ser. No. 11/955,895 (now U.S. Pat. No. 8,488,531 B2), filedDec. 13, 2007, which is a continuation of International PatentApplication No. PCT/CN2005/000857 filed on Jun. 15, 2005. Theafore-mentioned patent applications are hereby incorporated by referencein their entireties.

TECHNICAL FIELD

The present invention relates to the field of radio communicationsystems, and in particular to a method and system for allocatingcommunication resources, especially for packet-based, multi-usercellular communication systems.

BACKGROUND

In packet-based, multi-user cellular communication systems, such asmulti-user OFDM (Orthogonal Frequency Division Multiplexing) systems, ascheduler-device that makes decisions as to which user is assigned whichradio resources and when is typically employed. From time to time, usersreport the quality of their respective radio channels to the basestation, whereupon the base station makes a scheduling decision. Inuplink the base station measures the channel quality, e.g., from pilotsignals transmitted by the users. The scheduler may exploit the factthat the users' channels change independently from each other, i.e.,channels of one or more users may be fading, or, also, one or morechannels allocated to a specific user may be fading, while others arenot. Typically, a user is assigned radio resources when its channelconditions are good. Accordingly, the scheduler improves the performanceof the system (in terms of cell throughput) as compared to systems thatdo not exploit the users' channel quality through a scheduler.

The extent to which the scheduler improves the system performancedepends in downlink on the quality of the feedback information. Good,detailed and accurate feedback of the channel quality are necessary.There are situations, however, where such accurate and timely feedbackis not possible. A user may, for instance, move at such a high speedthat a channel quality measure is outdated and obsolete by the time itreaches the base station. Another example of unreliable feedbackmeasures occurs when a cell-edge user has bad signal-to-noise ratio onthe uplink feedback channel and the quality measure is simply detectederroneously at the base station.

Scheduling may also be inefficient if the channel varies considerably intime during a transmission time interval, i.e. in case of high Dopplerspread. For certain types of data scheduling may not be desirable, e.g.for data with low latency requirements and low data rates. Feedback datais a typical example of such kind of data. In this case, dedicatedchannels are more appropriate.

For these situations, the system may provide a frequency-distributedchannel in order to provide a high-diversity link-performance.Accordingly, users with reliable channel quality feedback and with dataappropriate for scheduling are assigned radio resources when and wheretheir respective channel conditions are known to be good, other usersare assigned frequency-distributed channels.

A problem, however, is how to provide both high link-diversity(frequency-distributed) channels and high multiuser-diversity(frequency-localized) channels at the same time in an efficient way.

An attempt to solve this problem is disclosed in IEEE Std 802.16-2004,“Standard for Local and Metropolitan Area Networks”, Part 16: “AirInterface for Fixed Broadband Wireless Access Systems”, 2004, whereinthe above problem has been solved through the use of so-called ‘zones’.A zone is a time period during which a certain type of channel istransmitted. Each radio frame contains two zones, one for thetransmission of frequency-localized channels followed by one forfrequency distributed channels in a pure time-multiplexing fashion. Inthe header of each radio frame information is conveyed as to when intime one zone changes into the next.

A disadvantage with this solution, however, is that the link-diversityis limited.

Accordingly, there is a need for a system and method with improvedlink-diversity.

SUMMARY

Embodiments of the present invention provide a system and a method forallocating communication resources in a multi-user cellularcommunication system, which has an improved link-diversity as comparedto the known prior art.

In accordance with the embodiments of the present invention,communication resources are divided in periods of time and frequencysub-bands, wherein part of the communication resources are used forfrequency-localized communication channels, and part of thecommunication resources are used for frequency distributed channels Themethod includes the steps of:

-   -   classifying part of said sub-bands as sub-bands carrying        frequency-distributed channels, and    -   classifying part of said sub-bands as sub-bands carrying        frequency-localized channels.

This has an advantage that frequency-localized communication andfrequency-distributed communication can be performed simultaneously andwithout interruption, since the potential delay associated with each ofthe two channel types, imposed by the ‘zone’-structure of the prior art,is eliminated. Further, since the present invention allows data to betransmitted continuously, the embodiments of the present invention havean advantage that link-diversity is improved because even if the signalquality is poor during part of a transmission frame, signal qualityduring the rest of the frame may be sufficient enough to ensure acorrect transmission. Even further, the embodiments of the presentinvention have an advantage that there is no delay until the desiredtype of channel (localized or distributed) is available, since bothtypes of channels always are available, which is a substantialadvantage, in particular for packet data transmissions with demands forfast retransmissions.

After performing the classification of the frequency sub-bands, theclassification may be changed from time period to time period, after acertain number of time intervals or at predetermined intervals. Whichsub-bands are of which type can be transmitted on a broadcast channel inthe beginning of, or prior to, a time period. This has the advantagethat a distribution of frequency-localized and frequency-distributedcommunication resources which optimises system throughput always can beused.

A code representing a particular arrangement of the communicationresources may be transmitted to the receiver, wherein the code may beused by the receiver to retrieve the communication resource scheme to beused. This has the advantage that rather complicated communicationresource schemes may be communicated to the receivers withoutsubstantial signalling.

The classification of the frequency sub-bands may be kept from timeperiod to time period. This has the advantage that the system may bestandardized, i.e. the sub-bands may always be used for a specific kindof communication.

The frequency-distributed channels may consist of frequency hoppingchannels, time or code multiplexed channels, or interleaved frequencymultiplexed channels This has the advantage that link-diversity of thefrequency-distributed channels may be increased even further.

The embodiments of the present invention also relate to a transmitterand a multi-user communication system.

Further advantages and features of the embodiments of the presentinvention will be disclosed in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art method of transmitting frequency-localised andfrequency-distributed communication.

FIG. 2 shows a communication resource scheme suitable for use with theembodiments of the present invention.

FIG. 3 shows an exemplary embodiment of the present invention.

FIG. 4 shows another exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

As described above, communication in a packet-based multi-usercommunication system can be performed using frequency-localizedchannels, i.e. channels which are assigned to users based on channelquality measurements. As also is stated above, in certain situations,such as when a channel quality measure is outdated and obsolete by thetime it reaches the base station due to a fast moving user, or whencommunicating data with low latency requirements and low data rates,frequency-localized communication may be undesirable. In such cases,communication using frequency-distributed channels may be preferable.

Accordingly, there is a need for a system utilizing both types ofcommunication.

When using frequency-localized and frequency-distributed channels, thechannels must be complementary (i.e., they must use disjoint resources)and yet as many of the physical radio resources as possible must beassigned to users (resources must not be unused).

FIG. 1 shows a prior art solution. As can be seen in the figure, thefrequency band is divided into sub-bands 10 a-10 j, each consisting of anumber of sub-carriers (not shown). Further, the allocation of thesub-bands are divided into transmission frames, of which one is shown,each consisting of a certain period of time, divided into time slotsTS1-TS8, which in turn are divided into a number of symbols. When it isdesired to have both frequency-localized and frequency-distributedcommunication, each frame is divided into two zones, of which the firstzone Z1 is used for the frequency-localized channels followed by asecond zone Z2 for frequency distributed channels in a puretime-multiplexing fashion. As stated above, information as to when onezone changes into the next is conveyed in the header of each radioframe. As stated above, a disadvantage with this solution is thatfrequency-localized communication and frequency-distributedcommunication cannot be performed simultaneously. Further, a delayassociated with each of the two channel types is imposed by the‘zone’-structure. Even further, time diversity of the frequencydistributed channels, is degraded since these channels are limited toonly one of the zones Z1, Z2.

In FIG. 2 is shown a communication resource scheme suitable for use withthe embodiments of the present invention. The disclosed system is amultiple-carrier OFDM system, having a two-dimensional structure (timeand frequency). The frequency spectrum of the OFDM system is dividedinto ten sub-bands 20 a-20 j, preferably constituting equal portions ofthe frequency spectrum. Equal frequency sub-bands are preferred tofacilitate resource management (for example, it is easier to allocatethe available resources). However, division into non-equal frequencysub-bands is, of course, also possible. Each sub-band is divided into anumber of sub-carriers, for example, each sub-band may consist of 20sub-carriers (not shown), however, sub-bands consisting of any number ofsub-carriers are possible, e.g., 1, 5, 100 or any other number.

In the time domain, the frequency spectrum is divided into time-slots,which typically has the length of a number of OFDM symbols. The figureshows one time slot consisting of eight OFDM symbols S1-S8. Thefrequency/time spectrum thus constitutes a communication resourcescheme, wherein, the smallest resource allocated to a user is onesub-band during one OFDM symbol (for frequency-distributedcommunication, as will be described below).

Instead of, as in the prior art, divide a transmission frame indifferent time zones, which are used for frequency-localized andfrequency-distributed communication, respectively, the communicationresource scheme is divided in frequency.

In this exemplary embodiment, the sub-bands 20 a, 20 c, 20 e, 20 g, 20 iare used for frequency-localized communication, while the sub-bands 20b, 20 d, 20 f, 20 h, 20 j are used for frequency-distributedcommunication. Further, as can be seen in the figure, there are threeusers UE1-UE3 communicating on the frequency-localized channels (UE1 onsub-band 20 e; UE2 on 20 c and 20 i; and UE3 on 20 a and 20 g), and twousers UE4, UE5 communicating on the frequency-distributed channelsAccordingly, the embodiments of the present invention allow that allusers may benefit from continuous data transmission, irrespective ofwhich type of communication that is utilized. Further, the continuoustransmission has the advantage that data transmission usingfrequency-distributed channels are spread all over time slot and thusalso all over a frame, and not just part of it, which improves timediversity of the channel Further, the embodiments of the presentinvention have an advantage that it increases the throughput in thesystem, since, for example, frequency-localised communication can alwaysbe performed, which, in turn, allows communication with users when theyhave a good channel quality, irrespective of when in, e.g., atransmission frame.

In use, a scheduler is used to multiplex the frequency-localizedchannels onto sub-bands determined to be used for frequency-localizedchannels, and frequency-distributed channels are multiplexed ontosub-bands to be used for frequency-distributed channels.

As is obvious to a person skilled in the art, any arrangement of thefrequency-localized and frequency distributed channels may be used. Inthe example shown in FIG. 1, each type of communication is allocatedhalf the resources. As is obvious to a person skilled in the art,however, the distribution of frequency-localized and frequencydistributed channels is arbitrary, as long as at least one sub-band isused for either of the two types of communication. For example, iftraffic in the cell varies in time, the channel distribution may vary intime as well. Preferably, however, the channel distribution is notchanged too rapidly, i.e. a number of frames in a row utilize the samedistribution, as this substantially reduces signalling in the system.The base station may transmit, on a broadcast channel, which sub-bandsare of which type, e.g. each time slot or each time there is a change inchannel distribution. As is common in a system utilizing frequencylocalized channels, a scheduler makes decisions as to which user isassigned which frequency localized channels, whereupon this informationis fed forward over a control channel to the users. Also, data regardingwhich user is assigned which frequency-distributed channels iscommunicated.

Data transmission on the frequency distributed channels may use varioustechniques for increasing diversity further. For example, as is shown inFIG. 2 for users UE4 and UE5, frequency hopping may be utilized. In thiscase, the base station and the users employ an algorithm to obtain theparticular frequency hopping sequence for a frequency hopping channelFurther, a user may be allocated two or more sub-bands forfrequency-distributed communication.

Even further, in one embodiment of the present invention, there are oneor more predefined channel resource schemes programmed in the basestation and the receivers. In this way, the base station can transmit acode representing which scheme to use, e.g. on a broadcast channel,whereupon the receiver can use a look-up table to obtain the channelarrangement of the particular scheme. Each time the channel resourcescheme is changed, the base station transmits the code representing thenew scheme. This has an advantage that this kind of signalling is keptto a minimum. In an alternative embodiment, the base station may signalwhich frequency sub-bands that are to be used for which kind ofsignalling. This may be effected, e.g., at predetermined intervals,and/or each time the category (frequency localized or frequencydistributed) of a sub-band is changed. As an even further alternative,the communication system standard may comprise only one configuration,which, accordingly always is used and thus has the advantage that nosignalling regarding the communication resource scheme arrangement isnecessary.

The use of frequency hopping algorithms may, as disclosed above, belimited to those sub-bands that are used for frequency-distributedchannels However, in a system consisting of a plurality of basestations, it is often preferred to utilize frequency hopping patternsthat in some way are optimised regarding to inter base stationinterference. If the employed frequency hopping patterns are generatedbased on the particular channel distribution of the base station, thefrequency hopping patterns of neighbouring base stations, which utilizesdifferent channel distributions, may disturb each other. Therefore, asis shown in the exemplary embodiment in FIG. 3, the frequency hoppingalgorithm may be generic in the sense that it applies to anyclassification of the sub-bands, however with the restriction thatsub-bands for frequency localized channels may not be part of thegenerated hopping sequence, hence these bands are eliminated from thehopping patterns in case the generating algorithm makes them appear inthe hopping sequence.

Generic frequency-hopping sequences have the following advantages. Itprioritizes the allocation of non-hopping (localized) channels, whichheavily depends on the channel quality, allowing arbitrary allocation ofthe localized channels based on channel quality. Moreover, hoppingpatterns that are designed for limited mutual interference can be usedwithout any modification, and with actual improvement of the inter-cellinterference between the hopping patterns. This is illustrated in FIG.3, wherein the hopping pattern for UE4 indicates that in TS2, sub-band30 a should be used, and in TS6, sub-band 30 e should be used.

In these time slots, however, according to the embodiments of thepresent invention, no data for UE4 is transmitted, but instead thefrequency-localized data for UE3 in TS2, and UE1 in TS 6, isprioritised.

This solution has the advantage that it reduces the downlink signalling,since the same frequency hopping patterns may be utilized irrespectiveof which channel distribution is utilized. Further, the balance betweenthe needs for frequency-distributed channels and frequency-localizedchannels can vary between cells and, in a certain cell, over time. It istherefore desirable that different cells can employ differentmultiplexing configurations in order to efficiently serve the presentusers. For the same efficiency reason, it is desirable that themultiplexing configuration in a cell can change over time.

FIG. 4 shows a further exemplary embodiment of the present invention.Here, the upper three sub-bands 40 a-c are used for frequency-localizedchannels, while the lower three sub-bands 40 d-f are used forfrequency-distributed channels In this embodiment, however, no frequencyhopping is used, instead channel diversity is accomplished usinginterleaved frequency multiplexing, i.e., users UE4-UE6 each areallocated channels in each of the sub-bands 40 d-f, i.e. each of theuses are allocated one or more sub-carriers in a sub-band. Further ways(not shown) to accomplish channel diversity includes time multiplexingand code multiplexing.

What is claimed is:
 1. Method for allocating communication resources forcommunication between a transmitter and a receiver, in a multi-usercellular communication system, wherein communication resources aredivided in time periods and frequency sub-bands, wherein part of thecommunication resources are used for frequency-localized communicationchannels, and part of the communication resources are used for frequencydistributed channels, the method comprises: classifying part of thefrequency sub-bands as frequency sub-bands carryingfrequency-distributed channels; and classifying the remaining part ofthe frequency sub-bands as frequency sub-bands carryingfrequency-localized channels.
 2. Method according to claim 1, furthercomprising after performing the classification of the frequencysub-bands: changing the classification from time period to time period,after a certain number of time intervals or at predetermined intervals.3. Method according to claim 1, further comprising transmitting, on abroadcast channel, in the beginning of, or prior to, a time period,which sub-bands are of which type.
 4. Method according to claim 1,wherein a code representing a particular arrangement of thecommunication resources is transmitted to the receiver, and wherein thecode is used by the receiver to retrieve the communication resourcescheme to be used.
 5. Method according to claim 1, further comprisingafter performing the classification of the frequency sub-bands, keepingthe performed classification from time period to time period.
 6. Methodaccording to claim 1, wherein the frequency-distributed channelscomprise frequency hopping channels.
 7. Method according to claim 6,wherein the frequency hopping channels are based on a generic hoppingsequence and wherein sub-bands used by non-hopping channels areeliminated from the hopping sequence.
 8. Method according to claim 1,wherein the frequency-distributed channels comprise one of timemultiplexed channels, code multiplexed channels, and interleavedfrequency multiplexed channels.
 9. Method according to claim 1, whereina scheduler is used to multiplex the frequency-localized channels ontothe frequency sub-bands determined to be used for frequency-localizedchannels, and wherein the frequency-distributed channels are multiplexedonto the frequency sub-bands to be used for the frequency-distributedchannels.
 10. Method according to claim 1, wherein the time period is atime slot or a transmission frame.
 11. System for allocatingcommunication resources for communication between a transmitter and areceiver in a multi-user cellular communication system, whereincommunication resources are divided in time periods and frequencysub-bands, wherein part of the communication resources are used forfrequency-localized communication channels, and part of thecommunication resources are used for frequency-distributed channels, thesystem comprises a device configured to: classify part of the frequencysub-bands as frequency sub-bands for carrying frequency-distributedchannels, and classify the remaining part of the frequency sub-bands asfrequency sub-bands for carrying frequency-localized channels. 12.System according to claim 11, further comprising a device configured tochange the classification from time period to time period, after acertain number of time intervals or at predetermined intervals. 13.System according to claim 11, further comprising a device configured totransmit, on a broadcast channel, in the beginning of, or prior to, atime period, which sub-bands are of which type.
 14. System according toclaim 11, further comprising a device configured to transmit a coderepresenting a particular arrangement of the communication resources tothe receiver.
 15. System according to claim 11, further comprising adevice configured to keep the performed classification from time periodto time period.
 16. System according to claim 11, wherein thefrequency-distributed channels comprise frequency hopping channels. 17.System according to claim 16, wherein the frequency hopping channels arebased on a generic hopping sequence, and wherein the system comprises adevice configured to eliminate sub-bands used by non-hopping channelsfrom the hopping sequence.
 18. System according to claim 11, wherein thefrequency-distributed channels comprise one of time multiplexedchannels, code multiplexed channels, and interleaved frequencymultiplexed channels.
 19. A transmitter for use in a in a multi-usercellular communication system, comprising a device configured to:classify part of frequency sub-bands as frequency sub-bands carryingfrequency-distributed channels, and classify the remaining part of thefrequency sub-bands as frequency sub-bands carrying frequency-localizedchannels.
 20. A multi-user cellular communication system havingcommunication resources for communication between at least onetransmitter and one receiver, comprising at least one transmitterconfigured to: classify part of frequency sub-bands as frequencysub-bands carrying frequency-distributed channels, and classify theremaining part of the frequency sub-bands as frequency sub-bandscarrying frequency-localized channels.